A faraday shield comprises a metallic cage or enclosure surrounding equipment or elements being protected from the effects of an electric field. An electric field set up outside the cage or enclosure has no effect on instruments or other electronic equipment located inside the cage or enclosure. A faraday shield generally requires some sort of pass-through element, such as a capacitor, in order for leads to extend into or out of the faraday shield to connect the electronic equipment inside the shield to equipment or systems located outside of the shield. It is imperative that no gaps exist between any screening and the leads associated with the shield. The pass-through elements can be used to prevent leakage paths into the enclosure while preventing shorting of the protected elements. However, the pass-through elements can be quite large and cumbersome thereby making the faraday shield large.
While effective for many applications, the size of the faraday shield may present a problem when the shield is applied to certain uses. This size drawback may even prevent use of a faraday shield in some situations. One such situation is in protecting electroexplosive devices (EEDs) from the effects of electric fields in the environment of the electroexplosive device (EED). Many uses of an EED require the EED to be smaller than presently available faraday shields will permit. One such use is in the mining and tunneling art. The uses of EEDs and the electromagnetic energy to which such EEDs might be exposed will be discussed below, with special emphasis being placed on the use of EEDs in the mining and tunneling industry.
An electroexplosive device is an initiator or a system in which an electrical impulse initiates detonation or deflagration of an explosive. An EED generally includes a power source electrically connected to an electrical initiation element (EIE) via input firing leads. The EIE is of the type which heats up when current is passed through it, and is in heat transferring contact with an explosive charge. When power is applied to the EIE, it heats up and eventually reaches a temperature which ignites the explosive charge contacting that EIE. Such EEDs are used in both the military market and in the civilian market for blasting applications, for ammunition applications as well as for air bags or the like. Because EEDs can rapidly generate large volumes of gas, they also can be used in conjunction with nearly any item which must be rapidly inflated.
As is well known to those skilled in the EED art, RF energy can cause an undesired actuation of an EED. RF energy incident on an EED can induce a current though the EIE of the EED. If the RF power level is high enough, or if the RF energy has a frequency which is high enough, the current induced in the EIE can become high enough to heat the EIE to a temperature which will activate the explosive charge thereby activating the EED in an undesired manner.
RF energy induced actuation of an EED is undesirable in any situation, and is especially undesirable if the EED is expected to be used in locations where there are high concentrations of high power RF and/or high frequency RF, such as near radar installations or the like. For this reason, many applications of EEDs, especially by the military, have extremely high standards for RF protection required for such EEDs.
As discussed in the incorporated documents, the EED art contains several devices which are intended to prevent undesired RF energy induced firing of an EED. Some of these devices include a Ferrite element surrounding the input leads. Some of these devices were discussed in the incorporated documents, and reference is made thereto for such discussion.
As discussed in these documents, the devices disclosed in the incorporated documents protect EEDs from inadvertent RF energy induced firing as well as from inadvertent electrostatic energy induced firing. These devices effect this protection by having an impedance path between the EED lead lines and ground which, for RF energy incident on the device, is lower than the impedance on the lead line to the firing element. Therefore, for a pin-to-case situation, current induced by RF energy induced potential is shunted to the case rather than flowing to the firing element via the lead line. Likewise, for a pin-to-pin situation, the current induced by RF energy-induced potential is simply dissipated as heat in the ferrite rather than flowing to the firing element. Therefore, the devices disclosed in the incorporated documents have shown outstanding protection features for EEDs that are exposed to high levels of RF and electrostatic energy. In fact, tests have shown that EEDs protected by the devices disclosed in the incorporated documents can be exposed to RF energy levels as high as fourteen watts at one megahertz and will prevent the EIE of the EED from being actuated by RF energy. Therefore, these devices have proved to be excellent protection against both RF and electrostatic energy.
As mentioned above, the mining and tunneling industry has long used EEDs. This industry has tried to protect the EEDs using several means, such as earthing devices, fuses, exploding bridgewires, frequency coded firing systems, or the like to protect the EEDs from inadvertent RF energy induced firing. While these devices have proved to be somewhat effective in limited situations, they still have several shortcomings that have generally limited their use.
In particular, the EEDs used in the mining and tunneling industry are subject to inadvertent detonation due to nearby lightning strikes, even lightning strikes occurring great distances from the EED, as well as due to inadvertent detonation due to RF energy generated by sources in the immediate vicinity of the mining or tunneling operation.
For example, some studies have concluded that surface lightning strikes occurring as much as twenty kilometers from the EED may generate currents that can endanger unprotected EEDs. As will be discussed in this disclosure, these stray currents can travel by several paths to reach the unprotected EED. Still further, other studies have concluded that high voltage power lines such as might be associated with an electrified trackway system, in the vicinity of a mining or a tunneling operation can generate sufficient RF energy to endanger unprotected EEDs used in the mining or tunneling operation.
In particular, in recent years, there has been a growing concern regarding safety-hazards due to nearby lightning effects in shallow coal mines. Several reported underground fatal accidents have been related to the passage of thunderstorms. Other more numerous incidents include electrical shocks, sparking from underground equipment, as well as premature and unexpected detonation of explosives in the charge faces and methane gas ignitions. These problems are of a serious nature, with a risk of serious injury or possibly death, to personnel in the vicinity of such an incident.
Presently, it is theorized that nearby lightning strikes and high voltage sources create adverse effects in a coal mine by several mechanisms:
(a) direct flow of lightning current through the rock formations surrounding a working coal face and in the face itself due to a strike close by; and/or
(b) the transfer of a high potential by metalwork in the mine from a more or less remote point (e.g., an above-ground conveyor gantry) to the working face, following a direct lightning strike to the above-ground steelwork.
A voltage of about 50 kV has been measured between a coal face and earthed metalwork in a colliery, due to nearby lightning strikes. On various occasions, men have received electric shocks from roofbolts or machines during lightning storms. Such roofbolt paths may also endanger unprotected EEDs located nearby. Detonators (sometimes a number of detonators wired individually or connected in series) have been fired simultaneously by lightning currents.
These are clearly undesirable situations.
Several important features of a typical coal mine as well as possible lightning disturbance mechanisms are illustrated in FIG. 1.
As seen in FIG. 1, three basic mechanisms are thought to be responsible for the energy transfer from the surface to the coal face. These are electromagnetic induction, direct conduction and direct strike. The energy transfer may result ultimately in a voltage surge propagating through the mine via any conductive system that is available, such as the conveyor system, water pipes, housing of electrical equipment, armoring of cables, or the like to an unprotected EED. Therefore, any EED used in the mining or tunneling operation must be protected from inadvertent firing induced by any of these just-mentioned mechanisms.
The surge may be attenuated as it propagates through the mine due to losses in transmission path. However, since contact between the conductive system and the rock strata is usually poor, the wave can still propagate to the farthest extremities of most mines. The transmission line terminations near the mining face are most likely to be open circuit in nature and the surge would typically be reflected and doubled. This would lead to an increased concentration of potentials thereby increasing the danger to unprotected EEDs.
The ways in which the travelling wave is initiated vary for the different mechanisms, and all may endanger EEDs or other electronic equipment used in a mining or tunneling operation. In the simplest case, i.e., a direct strike to a structure at a shaft entrance or ventilation shaft, a voltage surge is the only energy transfer mechanism involved. The local injection of lightning current into the earth via an electrode system causes the potential of the structure to rise in relation to a remote earth.
A more complex situation arises when there is a lightning strike on the surface away from the shaft. Here, electromagnetic induction and/or direct conduction are the relevant energy transfer mechanisms.
For the latter situation, the conductive path through the intervening rock and soil media is augmented underground by any of the conductive systems mentioned above. A voltage surge results from the steep potential gradients formed when the current distribution is distorted. Local variations in the terrain, such as dikes, boreholes, and geological faults, the mine excavations and the underground conductive systems can all locally distort the current and thus initiate a surge. A cased borehole would provide an excellent conductive path form the surface into the mine.
The third mechanism, i.e., electromagnetic induction, is possible since the electromagnetic fields associated with the discharge of the lightning channel are very high. These fields penetrate through the overlying ground into the mine tunnels. A current, and therefore a voltage surge, is induced into any conductive system situated in the electromagnetic field. In this case, energy may be transferred to the coal face not only via the propagating voltage surge but also through the electromagnetic fields themselves. In fact, a wave guide effect may be present since the coal seam, of high resistivity, is surrounded by shale of low resistivity. For the same reason, a capacitive effect may also be present.
As mentioned above, the voltage surges propagate along any conductive path, including the armorings of cables and housing of electrical apparatus. These are often operated in close proximity to the charged faces
In the case of the voltage surge, the conductive system will act as a radiating source, with the electromagnetic wave being radiated from its termination point. Where the electromagnetic fields are the only means of energy transfer, the lightning strike is the source. The amount of energy available for coupling into the detonator and exploder circuits depends on several factors. The rates of decay of the electric and magnetic field components with distance from the source are relevant. Thus, the position of the source is important. The field strength at any point also depends on the strength of the source (e.g., voltage surge magnitude) and the characteristics of the radiating and receiving antenna associated with the unprotected EED. The fact that the conductive system terminations are open circuit in nature is, therefore, very significant. The radiation, induction and quasi-static field components could all be important depending on the distance between source and receiver.
For all cases, the detonator and exploder circuits will act as "receiving antennas." The amount of energy extracted from the electromagnetic field will depend on various factors, including the conductor geometry, length and orientation relative to the incident field vector. In the case of a loop formed by the detonator wires, the area enclosed by the loop is also important. The energy will be maximum when the "receiving antenna" is made up of single wires, but will be greatly reduced by mutual coupling if the wires are twisted pairs construction. Accordingly, any EED design must account for this "antenna" effect as well, and not have the design protect against one form of energy transfer to the EED, while increasing the undesirable portions of the "antenna" effect.
A premature detonation will occur if the energy, i.e., current flowing into the circuit, is large enough to heat the detonator EIE. Any of these above-mentioned situations and conditions may create a situation in which an unprotected EED or other electronic equipment is subjected to sufficient energy to damage or inadvertently operate it.
The industry has therefore used several methods of combating risks underground due to currents caused by lightning. For example, all metalwork at shaft collars are usually earthed, parallel and adjacent metalwork are generally bonded underground, underground metalwork is usually maintained as close to the potential of the surrounding ground as possible by electrical bonding of that metalwork in groups to existing roofbolts installed to maintain a safe roof in the mine.
To combat lightning and certain induction or other stray current situations in collieries, it may even be necessary to establish earthing electrodes underground with sufficiently low impedance to earth.
While such devices and methods as discussed above have been somewhat effective, these devices and methods still have not been entirely effective in all situations and are not fully effective against protecting EEDs against RF-energy induced firing while maintaining an antenna profile that is as small as possible.
A faraday shield would vitiate, if not totally eliminate, this antenna effect while establishing effective protection for an EED. However, EEDs used in the mining and tunneling industry have size limitations. As mentioned above, the requirements for a pass-through capacitor element on a faraday shield prevents use of such devices to protect an EED in many mining and tunneling operations.
Therefore, there is a need for a means to protect electronic equipment such as EEDs, used in mining or tunneling operations from inadvertent damage or detonation caused by nearby lightning strikes or from RF energy. Specifically, there is a need for a faraday shield that is amenable to use in mining and tunneling operations.